将cassandra row映射到Spark RDD中的参数化类型

Question

我试图将cassandra行映射到使用spark-cassandra-connector的参数化类型。我一直在尝试使用隐式定义的columnMapper定义映射，因此:

class Foo[T<:Bar:ClassTag:RowReaderFactory] {
  implicit object Mapper extends JavaBeanColumnMapper[T](
    Map("id" -> "id",
        "timestamp" -> "ts"))
  def doSomeStuff(operations: CassandraTableScanRDD[T]): Unit = {
    println("do some stuff here")
  }
}

然而，我遇到了以下错误，我认为这是由于我传入RowReaderFactory而没有正确指定RowReaderFactory的映射。知道如何为RowReaderFactory指定映射信息吗?

Exception in thread "main" java.lang.IllegalArgumentException: Failed to map constructor parameter timestamp in Bar to a column of MyNamespace
    at com.datastax.spark.connector.mapper.DefaultColumnMapper$$anonfun$4$$anonfun$apply$1.apply(DefaultColumnMapper.scala:78)
    at com.datastax.spark.connector.mapper.DefaultColumnMapper$$anonfun$4$$anonfun$apply$1.apply(DefaultColumnMapper.scala:78)
    at scala.Option.getOrElse(Option.scala:120)
    at com.datastax.spark.connector.mapper.DefaultColumnMapper$$anonfun$4.apply(DefaultColumnMapper.scala:78)
    at com.datastax.spark.connector.mapper.DefaultColumnMapper$$anonfun$4.apply(DefaultColumnMapper.scala:76)
    at scala.collection.TraversableLike$WithFilter$$anonfun$map$2.apply(TraversableLike.scala:722)
    at scala.collection.immutable.List.foreach(List.scala:318)
    at scala.collection.TraversableLike$WithFilter.map(TraversableLike.scala:721)
    at com.datastax.spark.connector.mapper.DefaultColumnMapper.columnMapForReading(DefaultColumnMapper.scala:76)
    at com.datastax.spark.connector.rdd.reader.GettableDataToMappedTypeConverter.<init>(GettableDataToMappedTypeConverter.scala:56)
    at com.datastax.spark.connector.rdd.reader.ClassBasedRowReader.<init>(ClassBasedRowReader.scala:23)
    at com.datastax.spark.connector.rdd.reader.ClassBasedRowReaderFactory.rowReader(ClassBasedRowReader.scala:48)
    at com.datastax.spark.connector.rdd.reader.ClassBasedRowReaderFactory.rowReader(ClassBasedRowReader.scala:43)
    at com.datastax.spark.connector.rdd.CassandraTableRowReaderProvider$class.rowReader(CassandraTableRowReaderProvider.scala:48)
    at com.datastax.spark.connector.rdd.CassandraTableScanRDD.rowReader$lzycompute(CassandraTableScanRDD.scala:59)
    at com.datastax.spark.connector.rdd.CassandraTableScanRDD.rowReader(CassandraTableScanRDD.scala:59)
    at com.datastax.spark.connector.rdd.CassandraTableRowReaderProvider$class.verify(CassandraTableRowReaderProvider.scala:147)
    at com.datastax.spark.connector.rdd.CassandraTableScanRDD.verify(CassandraTableScanRDD.scala:59)
    at com.datastax.spark.connector.rdd.CassandraTableScanRDD.getPartitions(CassandraTableScanRDD.scala:143)

Answer 1

可以在Foo的伴侣对象中隐式定义，如下所示:

object Foo {
  implicit object Mapper extends JavaBeanColumnMapper[T](
    Map("id" -> "id",
        "timestamp" -> "ts"))
}

Scala在查找类的隐式实例时将查找类的伴侣对象。如果您愿意，可以在需要隐式的作用域中定义它，但您可能希望添加伴随对象，这样就不需要在必要时重复它。

Answer 2

证明columnMapper必须在创建Foo实例的范围内创建，而不是在Foo本身中创建。